This invention relates to the control of pressure in a roll, and more particularly relates to a method of and apparatus for controlling the linear pressure, or nip pressure distribution in a roll arrangement.
It has been proposed to control the linear pressure, or nip pressure, distribution in roll arrangements for the pressure treatment of webs of material. In a typical arrangement of this type at least one roll is provided which comprises a stationary beam extending axially of the roll and a shell rotatable therearound, the shell being supported internally on the beam with a number of supporting elements being located between the beam and the shell at positions disposed consecutively along the shell. The supporting elements can exert a pressure against the inner surface of the shell, thus controlling the linear, or nip, pressure exerted by the roll against a mating roll. In the prior proposed device a property of the web is measured after the web has passed through the nip and the measured value is compared with a reference value to produce controller adjusting signals which control the pressures exerted by the various supporting elements.
The pressure exerting supporting elements may be hydraulically operated.
In one particular prior proposed arrangement disclosed in No. DE-OS-2555677, stationary or mobile measuring elements are provided which extend transversely to the web, and which are adapted to detect particular properties thereof such as web thickness or web density or web moisture. The output from the measuring elements is fed to a computer which compares the actual measured properties with a programmed set of values which can be considered to be a property profile, representing the desired profile of the property being measured when considered transversely of the web. For example, the property profile may be a uniform distribution across the web. The computer produces the control or adjusting signals for adjusting the pressure exerted by the supporting members, thus serving to control the forces operative on the shell inner periphery, in response to the differences between the measured property and the programmed property profile.
The known system uses a roll assumed to be known per se from DE-AS No. 2230139. The supporting elements are hydrostatic pistons which are guided inclinedly in bores in the beam and which apply the linear or nip pressure. Systems disclosed by U.S. Pat. No. 3,119,324 and DE-OS No. 1461066 are also of interest. However, the term "supporting elements" is intended to be understood in the present context very broadly as denoting elements for local pressure control. Consequently, rolls according to DE-PS No. 2325721, wherein a piston acts on the side remote from the nip on the shell inner periphery, and rolls in accordance with DE-PS No. 2332861, wherein a piston is disposed in a chamber filled with hydraulic fluid and applies discrete additional forces, and rolls in accordance with DE-PS No. 3003395, in which no additional pressure is applied by a piston element but instead there is a chamber filled with a hydraulic liquid which exerts an overall pressure and means are provided for reducing the pressure, in selected locations, below the pressure of the surrounding hydraulic liquid, are suitable for the invention.
The control system known from DE-OS No. 2555677 (discussed above) starts from the assumption that the web has a particular width relatively to the roll. The pressures exerted by the discrete supporting elements are therefore calculated on the basis of this assumption. However, if this prior proposed system is used to process a web which is not of the desired width, the pressure control is wrong and the web is subject to an incorrect pressure profile.
For the purpose of explanation it will be assumed, as an example, that the web to be processed is narrower than the web originally intended. In this event parts of the roll which would engage the originally intended web now project beyond the edges of the narrow web and bend to some extent over such edges since the roll meets no opposing forces beyond the web edges. The forces applied to the shell in the outermost zones of the roll are applied additionally to the edge zones of the narrow web, and so there is a considerable increase in pressure at such edges. If webs which are not of the standard width are used, whilst the arrangement of the supporting elements in the roll still remains the same, the or each outermost supporting element is at times exerting pressure on parts of the web and at times exerting pressure on an empty space, depending on the width of the web being used at any specific instant.
The extent to which the applied pressure pattern at the edge of a narrow web varies from the required pressure pattern of a standard width web depends, of course, very greatly on the bending resistance of the shell of the roll. A very rigid shell retains its shape in the edge zone, even if no reactionary forces are being applied by the web, whereas a more flexible shell which is bent down beyond the edge of the web and is responsible for an abrupt increase in the pressure at the edge of the web.
It is relatively common for rolls of the kind used in the invention to have to deal with different web widths for instance. when webs from various origins have to be treated in converting or upgrading plants at locations separate from the manufacturing plants. The webs may be paper or plastics or other webs. At the present time it is only possible to treat webs of different widths on a particular machine by permitting departures from a predetermined linear pressure profile.